Documentation: fix incorrect macro referencing in mscc-phy-vsc8531.txt

Modify LINK_1000_ACTIVITY and LINK_100_ACTIVITY to VSC8531_LINK_1000_ACTIVITY
and VSC8531_LINK_100_ACTIVITY respectively in the example of ethernet-phy node
according to include/dt-bindings/net/mscc-phy-vsc8531.h.

Signed-off-by: Baisheng Gao <gaobaisheng@bonc.com.cn>
Link: https://lore.kernel.org/r/1627488086-200263-1-git-send-email-gaobaisheng@bonc.com.cn


Signed-off-by: Rob Herring <robh@kernel.org>